The present invention broadly relates to semiconductor device manufacturing equipment, and deals more particularly with a method and apparatus for dispensing a solution of photoresist used in the manufacturing process.
In processes for producing semiconductor devices from a semiconductor wafer, a number of techniques have been developed in order to form circuit patterns on the wafer. One of these processes employs photolithography which defines the circuit features on a wafer according to a specified pattern or mask. Subsequent manufacturing steps that are used to form a device include chemical and physical film depositions, etching, ion implantation, diffusion, annealing or thermal oxidation. Many of these processes require that a pattern of photoresist first be formed on the wafer substrate. The process for patterning the photoresist is referred to a photolithography process, which implies first depositing a uniform layer of photoresist onto the substrate, next exposing the photoresist layer to optical illumination through the mask, and then developing the exposed photoresist layer. The resist may be positive or negative, depending upon if it is to be removed or remain after the development of the irradiated regions. The development step may be carried out using wet chemical etching, dry plasma etching or by conversion to a volatile compound through the exposure radiation itself. The exposure radiation may be in the form of visible, deep ultraviolet or x-ray photons, or electron or ion beams of particles. The exposure can be made by a parallel process such as contact or projection printing from a mask, or by serially scanning one or more beams.
Prior to the application of the resist, the wafer surface must be cleaned, dehydrated and primed in order to improve the adhesion between the resist and the substrate. Cleaning steps are required because of the inevitable contamination which occurs during storage and handling between processing steps. As the minimum size of the circuit features is reduced, particles with diameters down to 10 nm and less have to be detected. Depending upon the expected contamination type and level, even chemical or dry cleaning will be required. Following surface cleaning, a dehydration bake is performed to evaporate most of the absorbed water. Once the wafer surface has been cleaned, a predetermined amount of resist is dispensed onto the wafer surface, either as the wafer remains motionless or is slowly rotated. Then, using a technique known as spin coating, the wafer is rapidly accelerated to a speed ranging between 2000 and 6000 RPM""s. The acceleration stage is crucial to obtaining good uniformity since the solvents within the photoresist begin evaporating from the photoresist immediately after dispensing. After about 30 seconds of spin time, less than one percent of the originally dispensed amount of resist remains on the wafer surface, with the remainder having flown off with the spinning process. The typical resist thickness range is from 0.05-1.5 microns, and may not vary more than plus or minus 5 nm across a flat wafer, and plus or minus 10 nm from wafer to wafer. At this stage, the resist has a tacky consistency since less than ⅓ of the solvent remains.
Dispensing systems typically include a reservoir of the resist solution coupled with a pump that pumps the solution from the reservoir to a nozzle where a discrete quantity is dispensed onto the wafer surface. The composition of the resist solution is such that air bubbles may become entrapped therein as the solution is pumped from the reservoir, through the pump to the dispensing nozzle. The introduction of air bubbles into the solution can result in an incorrect amount of the solution being dispensed, and can also result in the deposit of resist on the wafer containing these air bubbles. In either case, the dispensing of an incorrect amount of the resist or dispensing resist containing entrapped air bubbles can result in inconsistencies in the resist layer which in turn can cause defects in circuit features that cause malfunctions in the operation of a semiconductor device. Consequently, the presence of air bubbles in the solution increases the likelihood of scrap and therefore decreases process yield of good devices.
In an attempt to eliminate air bubbles from the resist solution prior to the dispensing process, prior dispensing systems have employed a trap tank coupled between the solution reservoir and the pump. This tank acts as a holding tank where the air bubbles are allowed to settle out and/or are drained off from the solution.
In addition to the problems discussed above, air bubbles can become introduced into the resist solution as a result of leaks in the piping system connecting the reservoir, tank trap and pump. Moreover, in the event that the reservoir runs empty, the pump sucks air into the feed lines for a period of time until solution is added to the reservoir. During this period, the pump effectively draws air into the lines which becomes mixed with the solution. In each of these events, in addition to reducing process yields, valuable processing time is lost because human intervention is necessary to correct the problem and in some cases, the entire processing operation must be suspended while operators flush the lines and refill them with a fresh resist solution.
It would therefore be desirable to provide a resist dispensing system that virtually eliminates the possibility of air bubbles becoming entrapped in the solution, and drawn through the dispensing pump. The present invention is directed toward solving this problem.
According to one aspect of the invention, apparatus is provided for dispensing a liquid resist solution used in a semiconductor device manufacturing process. The apparatus includes a reservoir of the resist solution, a trap tank for trapping air bubbles entrapped in the solution, a pump for drawing the resist solution from the tank to a dispensing nozzle, and a sensor for sensing the presence of air bubbles in the resist solution flowing from the tank trap to a dispensing nozzle and a sensor for sensing the presence of air bubbles in a resist solution flowing from the tank trap to the pump. A controller receives signals from the bubble sensor and controls the operation of the dispensing process. The sensor preferably comprises a light source for directing light through the resist solution, and a photosensitive detection element for detecting changes in the light passing through the resist solution and originating from the light source.
According to another aspect of the invention, apparatus is provided for use with a plurality of processing stations for processing semiconductor wafers which monitors the presence of air bubbles in resist solution drawn from a tank to a dispensing pump. The apparatus comprises a plurality of sensors for respectively detecting the presence of air bubbles in the resist solution drawn from the associated tank to the corresponding pump, wherein each of the sensors produces a signal indicative of the presence of air bubbles in the solution, and a circuit connected with each of the sensors for receiving the signals produced by the sensors and for generating an alarm signal then the circuit receives a signal from any one of the sensors. The circuit preferably includes a logic circuit having a plurality if inputs for receiving the sensor signals, and an output for delivering a trigger signal, and a switch actuated by the trigger signal for outputting the alarm signal.
According to still another aspect of the invention, a method is provided for dispensing liquid resist solution used in a process for manufacturing semiconductor wafer devices. The method includes the steps of flowing the solution from a reservoir into a tank; removing at least some of the air bubbles contained in the resist solution within the tank; flowing the solution from the tank through a line to a dispensing nozzle; sensing the presence of air bubbles contained in the solution flowing through the line; dispensing the solution through the nozzle; and, issuing an alarm when air bubbles are sensed. The sensing step is performed by passing light through the solution flowing through a line and sensing changes in the intensity of light passing through the solution. The changes in the light intensity being indicative of the presence of air bubbles in the solution.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for dispensing resist solution onto a semiconductor wafer that increases process yield and decreases equipment down time.
Another object of the invention is to provide a method and apparatus as mentioned above that provides early detection of air bubbles entrapped within the solution before the solution enters a dispensing pump.
A still further object of the invention is to provide a method and apparatus as described above which employs an interlock circuit cooperating with process control equipment that interrupts the operation of the equipment when air bubbles in the resist solution are sensed, to thereby prevent the dispensing of resist solution containing air bubbles.
These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the invention.